1. Field of the Invention
The present invention relates to a copolyester having high flowability and improved moisture and hydrolysis resistance comprising a copolyester which exhibits anisotropy in a molten state and has a functional group located at the terminal of its molecular chain and is blocked with a low-molecular compound having at least one aromatic ring.
2. Prior Art
In recent years, a liquid crystal polymer which exhibits anisotropy in a molten state has attracted attention as a resin having high strength, high heat resistance, and excellent chemical resistance as well as being characterized with excellent moldability and processability properties.
Various proposals have been made in connection with liquid crystal polymers. With respect to representative examples of such proposals, reference may be made to Japanese Patent Laid-Open Nos. 72393/1974, 43223/1975 and 50594/1979. In all of these liquid crystal polymers, a rigid monomer is incorporated into the skeleton thereof to develop liquid crystal properties, which characterizes a resin formed therefrom with a high strength and easy as is processable properties. As well known, a resin which exhibits anisotropy in a molten state has an extremely low viscosity because the melt viscosity depends on shear, which made it possible to utilize the resin per se as in the form of a thin film and fine parts or in the form of a composite material containing a filler and a reinforcing material incorporated therein for various heat-resistant and precision parts. In particular, the low viscosity of the polymer per se enables the incorporation of a filler and a reinforcing material in high proportions, which directs attention to the use of the resin for applications where an extremely high dimensional stability is required or in the field of magnetic materials in which the performance is proportional to the content of the filler.
In the above-mentioned resin, the molecular weight is usually reduced when a lower viscosity in a molten state is desired. Such an expedient somewhat spoils the mechanical strength, heat resistance, and chemical resistance of the resin. Particularly, since the bond units of the molecular chain comprise ester bonds, the resistance to hydrolysis greatly depends upon the molecular weight. Specifically, a reduction in molecular weight for the purpose of further reducing the viscosity of the polymer which exhibits anisotropy in a molten state, remarkably spoils the hydrolysis resistance and thus leads to the lowering in mechanical strength.
Although it is self-evident that the terminal of the molecular chain must be blocked in order to improve the hydrolysis resistance of a polyester, it is difficult to block the terminal and still not reducing the liquid crystal properties and lowering the heat resistance. This is because, not only is the development of the liquid crystal properties itself dependent on a delicate balance among various molecular skeletons, but also because the influence of the terminal blocking agent is likely to be more markedly exhibited than in the case of other polymers, due to the lowering in the molecular weight.
The present inventors have made extensive and intensive studies with a view toward realizing a resin having a combination of low viscosity with excellent hydrolysis resistance. As a result, the present inventors have found that the above-mentioned two properties which are contradictory to each other can be simultaneously realized by blocking the molecular chain terminal with a specific compound, which led to the completion of the present invention.